1. Field of the Invention
The present invention relates to a diffusion plate which is used, for example, as a focusing screen in a single lens reflex camera, and a method for manufacturing a master die for such a diffusion plate.
2. Description of Related Art
In a known single lens reflex camera or the like, a focusing screen is located at a position that is optically equivalent to a film plane, so that a photographer can compose an image, having a desired focus, by observing the image through a view finder.
It is known to make the focusing screen using a diffusion plate having microscopic projections and indentations (i.e., an uneven surface) to observe an unsharp image formed on the focusing screen (i.e., diffusion plate), owing to the characteristic of the diffusion plate to diffuse light.
In a known method to prepare such a diffusion plate, the outer surface of a master die is subject to sanding or sand blasting to form micro projections and indentations thereon which are then transferred or copied onto a plastic optical element of acrylic resin, etc.
The diffusion plate thus prepared has micro projections and indentations of irregular shape forming a combination of micro prisms having acute apex angles. Consequently, part of the light incident upon the diffusion plate, from a taking lens side, is refracted or bent by the apexes at acute angles. Accordingly, much of the light is lost through the view finder before reaching the photographer's eye. Furthermore, when a diaphragm is stopped-down, the granularity of the diffusion plate becomes visible, resulting in a poor image quality.
To eliminate the drawbacks mentioned above, it is known to prepare a master die for a diffusion plate with an uneven surface having a regular micropattern of smooth apexes using optical means, such as a photoresist process, instead of mechanical means, such as sanding, sand blasting etc. A molding die is manufactured by copying the master die using an electro forming process, so that the regular micropattern can be transferred to an optical element (diffusion plate), as disclosed for example in Japanese Unexamined Patent Publication No. SHO 55-90931 or SHO 57-148728.
As is well known, the diffusibility is represented by a Fourier transform of a transmission function of a diffusion plate. The transmission function f(x, y) of a conventional diffusion plate having a regular (two dimensional periodical) pattern is obtained by; ##EQU1##
wherein g(x, y) designates the transmission function of a microstructure, and p=(p.sub.x, p.sub.y) and q=(q.sub.x, q.sub.y) the two dimension periodic lattice vectors, .delta. the Dirac's .delta. function, and ** the two dimensional convolution integration, respectively. In this specification, p, q, and r, where referred to, represent vectors.
The Fourier transform f(.omega..sub.x, .omega..sub.y) of f(x, y) is given by the following discrete function. ##EQU2##
wherein G(.omega..sub.x, .omega..sub.y) designates the Fourier transform of the function g(x, y), and (a.sub.1, b.sub.1) and (a.sub.2, a.sub.b) the two dimensional lattice vectors of the diffusibility.
There is a relationship between (p.sub.x, p.sub.y) and (q.sub.x, q.sub.y) as follows.
D=a.sub.1 b.sub.2 -a.sub.2 b.sub.1 =(-p.sub.x q.sub.y -p.sub.y q.sub.x).sup.-1 PA1 a.sub.1 =p.sub.y D, b.sub.1 =-p.sub.x D, a.sub.2 =q.sub.y D, b.sub.2 =q.sub.x D PA1 .omega..sub.x and .omega..sub.y are given by; PA1 .alpha.=.lambda..omega..sub.x, .beta.=.lambda..omega..sub.y, .gamma.=(1-.alpha..sup.2 -.beta..sup.2).sup.1/2
wherein (.alpha., .beta., .gamma.) designates the direction cosine in the observation direction of the diffusion, and .gamma. the wavelength, respectively.
Therefore, the conventional diffusion plate having a regular (two dimensional periodical) pattern functions as a diffraction grating. Accordingly, the discrete diffusibility is inevitable. Consequently, an off-axis aberration, in which multi-lined images appear when a defocused (out of focus) image is viewed, takes place. Furthermore, since the diffraction angle varies depending on the wavelength, if the pitch (i.e., lengths p and q of the two dimensional lattice vectors) is small and the diffraction angle is large, there is a conspicuous irregularity in color of the observed image.
It is possible to prevent the multi-lined image and irregularity of color by increasing the pitch, but the increased pitch causes a periodic structure of the mat to appear within the field of view of the view finder, obstructing the view.
FIG. 1 shows a known diffusion plate having a pattern of microlenses having a maximum density arrangement. The microlenses are arranged at a pitch of 16 .mu.m, for example. Each microlens has 10 .mu.m diameter and 1.6 .mu.m height.
FIGS. 2 through 10 show diagrams of various optical properties of the diffusion plate shown in FIG. 1.
FIGS. 2 through 4 show diffusion properties (diffraction patterns) of the plate by showing defocused images of point light sources having wavelengths of 450 nm, 550 nm, and 650 nm, respectively. In FIGS. 2 through 4, the diameters of the small circles (points) g correspond to the intensity of diffracted light in the directions of the diameters, and the large circles h represent the F numbers of the bundles of rays incident on the diffusion plate, i.e., 2.0, 2.8, 4.0, 5.6, and 8.0, as viewed from the side of the outermost circle, respectively.
FIGS. 5 through 7 show the quantity of light (at wavelengths of 450 nm, 550 nm, and 650 nm, respectively) (represented by the ordinate of the graph) contained in the large circles (encircled power) depicted in FIGS. 2-4, respectively. The circles have radii represented by the values on the abscissa of the graph. The above assumes that the total quantity of light transmitted through the diffusion plate, as shown in FIG. 1, is 1.0.
FIGS. 8 through 10 show defocused image intensities of the line light sources (line diffraction patterns in which the longitudinal line light source is represented by the solid lines and the lateral line light source is represented by the imaginary lines) at wavelengths of 450 nm, 550 nm, and 650 nm, respectively. The abscissa represents the radii of diffraction of the diffraction patterns (diffusion angle of the diffusion plate), and the ordinate represents the relative intensity of the light, on the assumption that the peak intensity is 1.0, respectively.
FIG. 11 shows a structure of another known diffusion plate, having a square arrangement of microlenses (microstructures) having 10 .mu.m diameter and 1.6 .mu.m height at a uniform pitch of 16 .mu.m.
FIGS. 12 through 20 show diagrams of various optical properties of the diffusion plate shown in FIG. 11.
FIGS. 12 through 14 show diffusion properties of the plate by showing defocused images of a point light having a wavelength of 450 nm, 550 nm, and 650 nm, respectively. In FIGS. 2 through 4, the diameters of the small circles (points) g correspond to the intensity of diffracted light in the directions of the diameters, and the large circles h represent the F numbers of the bundle of rays incident on the diffusion plate, i.e., 2.0, 2.8, 4.0, 5.6, and 8.0, as viewed from the side of the outermost circle, respectively.
FIGS. 15 through 17 show the quantity of light (represented by the ordinate) contained in the circles having radii represented by the abscissa, at the wavelengths of 450 nm, 550 nm, and 650 nm, on the assumption that the total quantity of light transmitted through the diffusion plate, as shown in FIG. 11, is 1.0.
FIGS. 18 through 20 show defocused image intensities of the line light sources (longitudinal line light source represented by the solid lines and lateral line light source represented by the imaginary lines) at the wavelengths of 450 nm, 550 nm, and 650 nm, respectively. The abscissa represents the radius of diffraction, and the ordinate represents the relative intensity of light, on the assumption that the peak intensity is 1.0.
As can be seen from the drawings discussed above, the known two-dimensional periodic diffusion plates have a discrete diffusion property which largely depends on the wavelength, resulting in a deteriorated aberration property and irregularity in color.